1. Field of the Invention
The present invention relates to a coil device functioning as a transformer or choke coil used in various electronic circuits, and more particularly to a coil device having an edgewise winding of a rectangular insulated wire.
2. Description of the Related Art
Since, in recent years, the miniaturization and higher performance of an electric apparatus have been required, a coil device such as a transformer or line filter is also required to be miniaturized and be higher in performance. Conventionally, a round insulated wire has been used for a winding in the coil device, but an edgewise winding of a rectangular insulated wire has the following advantages over the winding of the round insulated wire, and is increasingly often used. Firstly, the edgewise winding can achieve higher performance, higher efficiency, miniaturization and lower-profile due to its larger conductor occupation ratio. Secondary, the edgewise winding has a smaller stray capacity and therefore can realize better frequency characteristics. Thirdly, the edgewise winding does not require a process of winding a wire on a bobbin, and is easier to assemble, resulting in an easier automation of the manufacturing process.
A coil device, which uses the above-mentioned edgewise winding, comprises, for example: a case which is formed of resin, and which includes a case body and case lid; and a coil which comprises a core and a winding formed of a rectangular insulated wire wound edgewise around the core and having lead wires, and which is disposed in the case body. In the coil device thus structured, at least one lead wire each winding is fitted in a groove formed on the inner surface of the case body, and a harness lead wire is inserted in a hole formed in the lead wire and soldered. The coil device is disclosed in Japanese Patent Publication No. Hei 10-241955, and refer to FIG. 6 thereof.
A surface-mount choke coil is disclosed in Japanese Patent Publication No. Hei 08-236364, which is formed such that a rectangular insulated wire is wound edgewise around the center leg of an EE-core, and both ends of the edgewise winding lead out respectively at sides of the core opposite to each other, and that the both ends leading out have their insulation peeled off to form flat connection portions (refer to FIG. 3 of the Japanese Patent Publication No. Hei 08-236364).
A surface-mount compact coil device is disclosed in Japanese Patent Publication No. Hei 11-345721, which comprises a pair of cores formed of magnetic material, at least two windings, and a bobbin, and in which at least one of the at least two windings is an edgewise winding of a rectangular insulated wire, and the edgewise winding and a flat winding are stacked with the core disposed inside the stacked windings (refer to FIG. 1 of the Japanese Patent Publication No. Hei 11-345721).
In the various types of edgewise windings used in the above-described coil devices, the starting and finishing ends of the rectangular insulated wires are usually soldered directly to a printed circuit board (hereinafter referred to as “PCB”). For example, referring to FIG. 5 showing a conventional coil device, staring and finishing ends of a rectangular wire leading out so as to work as terminals are let through holes formed in a PCB (not shown) and each shaped substantially rectangular corresponding to the cross-sectional configuration of the wire, and referring now to FIG. 6 showing another conventional coil device, starting and finishing ends of a rectangular wire are worked upon to come up with a structure suitable for surface-mount on a PCB taking advantage of its rectangular cross-sectional shape.
As shown in FIGS. 5 and 6, a holder 2 comprises two side walls 18 and 19 disposed respectively on its both sides, and two slot holes 4 and 4 for engagingly receiving a core securing plate spring 3 are each formed in the upper portion of each of the side walls 18 and 19. A core 5 is inserted in an edgewise winding 6 of a rectangular insulated wire, then is mounted on a base section 10 of the holder 2, and is secured thereto with the plate spring 3. End portions (terminations) 61 and 62 of the edgewise winding 6 have their insulation resin peeled off, are plated with solder, and lead out extending along respective wire paths 11 and 11.
In the coil device shown in FIG. 5, the terminations 61 and 62 are let through the aforementioned holes which are formed in the PCB (not shown) and which are shaped substantially rectangular corresponding to the cross-sectional configuration of the rectangular wire. On the other hand, in the coil device shown in FIG. 6, the terminations 61 and 62 are bent at respective corners of the base section 10 for enabling the coil device to be surface-mounted on the PCB (not shown). However, a terminal configuration of the terminations 61 and 62 shown in FIG. 5 causes such a problem that the coil device comes off from the PCB due to a mechanical vibration when the coil device is relatively high or heavy, thus limiting the size and weight of the coil device, and a terminal configuration of the terminations 61 and 62 shown in FIG. 6 comes up with a problem on a mechanical strength of the rectangular wire itself when the wire has a relatively small current capacity (thin wire) or the coil device is heavy. In the conventional terminal configurations, the coil device, such as a transformer or choke coil, mounted on the PCB must have its size and weight limited in order to stay stably secured thereon. And, since a conventional coil device usually uses a wire having a round cross section, a through-hole in a PCB for attaching the coil device also has a round cross section, which is different from the cross section of the aforementioned rectangular wire, thus requiring a change in designing the PCB in order to manufacture another PCB for enabling substitution by the rectangular wire, which causes difficulties in the substitution.